Biraha aan la midka ahayn waxay tixraacaan biraha kala duwan (sida aluminium, naxaas, iwm.) ama alloys gaar ah oo laga sameeyay isla birta aasaasiga ah (sida birta kaarboon, birta, iwm.) kuwaas oo leh farqi weyn oo u dhexeeya sifooyinka birta, sida jirka. quman...
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